Defects in the form of open lines or shorts are prevalent in non-volatile memories such as NAND flash memories. These defects can be present at the time of manufacture, or can be generated through the operating life of the memories. The defects can also be caused by regular wear and tear (e.g., periodic refresh operations over a long time). These defects contribute towards Raw Bit Error rate (RBER). The term “RBER” here generally refers to the rate of errors when reading from a non-volatile memory. If Low-Density Parity-Check Code (LDPC) is used to encode input data which is stored in the NAND flash memory, and when a soft read operation is performed, the open lines or circuits generally referred here as “opens”) the read memory may manifest as errors with high confidence (or errors with high reliability value, where reliability is the magnitude of the Log Likelihood Ratio (LLR)).
Having open circuits or open lines means that the memory cell cannot be used to store data. These high confidence errors are more detrimental to LDPC performance than soft errors. One reason for such detriment is that soft errors can allow LDPC decoding to continue (and the decoded result has a high chance of being corrected by error correction schemes) while hard (or high confidence errors) may cause LDPC decoding to fail with low or no chance of being corrected by the error correction schemes.